1. Field of the Invention
This invention relates to a process for forming a single crystal silicon layer on a single crystal silicon substrate in accordance with the epitaxial growth technique.
2. Description of the Prior Art
As a process for forming a single crystal silicon layer on a single crystal silicon substrate by making use of the epitaxial growth technique for fabrication of a semiconductor device, it has been known to form a starting silicon layer made of amorphous silicon (hereinafter called "a-silicon") or polycrystalline silicon on a single crystal silicon substrate and then to heat the starting silicon layer so as to allow the starting silicon layer to regrow into a single crystal layer having the same crystal plane as the single crystal of the substrate. For example, it has been known to heat a wafer, in which an a-silicon layer of for example 4000 .ANG. thick is formed on a surface of a single crystal silicon substrate, for example at about 600.degree. C. and approximately for 80 minutes in a resistance furnace so that the a-silicon layer is converted into a single crystal layer. The above process however requires to continue the heating over a relatively long period of time and is thus impractical from the viewpoint of productivity. It may be possible to increase the growth speed of each single crystal by raising the heating temperature in the above process. However, use of a high heating temperature in the above process is accompanied by such drawbacks that wafers tend to develop "warpage" or "contamination" and the production yield hence becomes poor.
On the other hand, it has recently been studied to develop a process for forming a single crystal layer by directing a laser beam for a short period of time onto a starting silicon layer formed on a single crystal silicon layer to melt the starting silicon layer and then solidifying the thus-molten silicon layer in accordance with the epitaxial growth technique. Reference may for example be made to U.S. Pat. No. 4,309,225. In the above process which makes use of a laser beam, an a-silicon layer is scanned by the small spot light of the laser beam. It has been pointed out, as a shortcoming of the above process, that the laser beam develops certain excessively-heated portions in the boundary region between each scanning line, along which the small spot light of the beam passes, and its adjacent scanning line and disturbs the crystalline texture there, thereby resulting in formation of grain boundaries; or the above process requires too much time if the interval of the scanning lines is shortened. Therefore, this laser beam process is difficult to form single crystal silicon layers which make up SOI (Silicon on Insulator) structures useful for fabrication of layered three-dimensional IC devices which are considered to incorporate "the latest IC system".